The present invention relates to a processing apparatus with a movable processing tool such as a cutting tool, a milling cutter, and a polishing or lapping tool which has an NC table.
An NC cutting apparatus with an NC table has been widely used lately in order to process parts such as video cylinders or the like with high accuracy.
A conventional NC cutting apparatus of the type referred to above will be described below.
FIG. 16 is a schematic plane view showing the structure of a conventional NC cutting apparatus, in which element 1 is an NC cutting apparatus and element 2 is a main shaft. A cutting tool 3 of the apparatus 1 is fixed to an NC table 4 by a means (not shown). A control device 5 controls the cutting apparatus 1 and the NC table 4. A chuck 6 mounted to the main shaft 2 holds a work 7 to be cut. It is to be noted here that the work 7 is supposed to be a video cylinder herein so as to make the point clear, and the apparatus is used to fix a shaft 8 of different material which is the rotational center of the work 7 through shrinkage fitting or pressure fitting as shown in FIG. 17.
The NC cutting apparatus 1 in the aforementioned structure operates in a manner as described below.
The work 7 is rotated in a direction of an arrow I following the rotation of the main shaft 2. At this time, the NC table 4 to which the cutting tool 3 is secured is moved in the directions of arrows II and III in accordance with a program preliminarily loaded into the control device 5. Accordingly, the cutting tool 3 bites the work 7, thereby cutting, for example, the outer peripheral surface of the work 7 with high accuracy.
In the above-depicted arrangement, however, although the work 7 itself is processed highly accurately, the shaft 8 is disadvantageously inclined to the outer peripheral surface of the work 7 as it is firmly secured to the work 7, as is clearly shown in FIG. 17. The actual inclination of the shaft 8 is on the order of several .mu.m, but exaggerated in FIG. 17 for the sake of explanation. Since the video tape of a video tape recorder as a final product runs along the outer periphery of the work 7 and moreover, the head part of the video tape recorder rotates around the shaft 8, it is impossible for the video head to correctly trace the video tape when the shaft 8 is inclined, thus resulting in the deterioration of the image quality. Even though this problem would be solved by processing the outer peripheral surface of the work 7 while using the shaft 8 as a processing reference after the shaft 8 is secured to the work 7, it is necessary in the conventional NC cutting apparatus to adjust and set the work 7 by the chuck 6 so that the outer periphery of the inclined shaft 8 is not deflected due to the rotation of the main shaft 2. The adjustment is however a manual work and has a very low efficiency for mass-production. Although the foregoing description is based on the assumption that the work 7 is a video cylinder, the problem is not peculiar to video cylinders. For instance, supposing that the outer periphery of a rod-shaped work 9 of FIG. 18 is to be processed accurately, a part 9b of the work 9 is first processed while a part 9a of the work 9 is held by the chuck 6, and then the part 9a is processed while the part 9b is supported by the chuck 6. When the part 9b is supported by the chuck 6, however, the rotational center of the main shaft 2 is minutely deflected from the center of the part 9b held by the chuck 6, generating a deflection on the order of .mu.m at a part where the part 9b is not held by the chuck 6. In the meantime, the part 9a is naturally processed at the rotational center of the main shaft 2. As such, it is a commonly-accepted idea that it is impossible to accurately process over the length of the rod-like work 9 by the conventional arrangement. This problem may also be solved in the same manner as in the case of the above video cylinder. In other words, the part 9b is first processed sufficiently longer than it is to be held by the chuck 6, and subsequently the part 9b is adjusted by the chuck 6 in a manner to prevent a part of the part 9b longer than the chuck 6 from deflection when the part 9b is held by the chuck 6. However, this solution alike is poor for mass production because of the dependency on the need for manual assistance.